1. Field of the Invention
This invention relates to a gas-filled discharge tube and more particularly to a gas-filled discharge tube which prevents internal flashovers and assures production with high yields even if the machining precision is not so high.
2. Description of the Prior Art
Gas-filled discharge tubes, in which a sealed tube is filled with a gas and a voltage is applied across the electrodes provided at each end of the sealed tube to cause a discharge, have been in use in a variety of fields.
FIG. 3 shows a conventional gas-filled discharge tube 1. A cylindrical member 2 formed of an electrically insulating material such as ceramics has its open end portions metallized. The open end portions of the cylindrical member 2 are sealed with a pair of discharge electrodes 4, 4'--each of which consists of a metal electrode plate 4a and a discharge electrode portion 4b--by means of solder 3. The cylindrical member 2 is sealed with an inert gas such as argon gas at a pressure as high as several to twenty atmospheric pressures. Application of a certain voltage difference between the discharge electrodes 4, 4' causes an electric discharge in a gap G between the facing discharge electrode portions 4b 4b' that have the steepest potential gradient.
In such a gas-filled discharge tube 1, it is known that the discharge characteristics are affected by the environment in which it is used. For example, if near the discharge tube 1 there is a grounded body (not shown) which is at the same potential as the discharge electrode 4' that is not applied with a voltage, a steep potential gradient may occur at other locations than the gap G between the discharge electrode portions, such as a joint portion between the cylindrical member 2 and the other discharge electrode 4 that is applied with a voltage. In such a case, as shown in FIG. 3, partial discharge glows or coronas 5 along the inner surface occur prior to the discharge between the discharge electrode portions 4b, 4b'. The partial discharge glows 5 develop into an internal flashover discharge 6 that envelops the entire inner surface of the cylindrical member 2. As a result, a starting voltage of a discharge required for the discharge gap cannot be obtained, changing the discharge characteristics of the discharge tube.
To eliminate this problem, it has been proposed to form a flange 7 integrally with and almost at the middle portion of the inner surface of the cylindrical member 2 in order to elongate the distance along the inner surface of the cylindrical member 2 between the discharge electrodes 4, 4', as shown in FIG. 4, thereby making the flashover along the inner surface difficult to occur.
With the discharge tube 1 shown in FIG. 4, however, since the flange 7 is formed projecting in a space A surrounding the gap G between the facing discharge electrode portions where the field intensity is strong, the electric field generated between the discharge electrode portions 4b, 4b' is disturbed by the flange 7, changing the discharge characteristics of the discharge tube 1.
To overcome this drawback, another method has been proposed. According to this method, as shown in FIG. 5, the flange 7 is formed integrally with one end portion of the cylindrical member 2 so as to project in a space B, which is remote from the gap G between the opposing discharge electrode portions so that the field intensity is not so strong as to affect the discharge characteristics. In the cup-shaped cylindrical member 2, one of the discharge electrodes 4 is sealed to the end surface of the flange 7 while the other discharge electrode 4' is sealed to the other end surface of the cylindrical member 2.
The above-mentioned conventional gas-filled discharge tube 1, however, has drawbacks. For example, where the cylindrical member 2 is formed of ceramics, it may be warped while being sintered. Further, as shown in FIG. 6, at the sealing portions between the cylindrical member 2 and the discharge electrodes 4, 4', the contact length x between one discharge electrode 4 and the end surface of the flange 7 is longer than that between the other discharge electrode 4' and the end surface of the cylindrical member 2. This increases a gap t formed between the first discharge electrode 4 and the flange 7, making it impossible to perfectly seal the gap between the discharge electrode 4 and the end surface of the flange 7 by solder 3. This in turn will lead to a possible leakage of the inert gas from the cylindrical member 2, lowering the yield during the manufacturing process.
Another type of discharge tube is available. As shown in FIG. 7, the discharge electrodes 8, 8' sealed to the end surfaces of the cylindrical member 2 are formed by press-forming thin metal plates. In addition to the warping of the cylindrical member 2 caused by sintering process, this type of discharge tube has another problem that the discharge electrodes 8, 8' may also be distorted while being press-formed. This causes the gap t formed between one of the discharge electrodes 8 and the flange 7 of the cylindrical member 2 to increase.
In either type of the above-mentioned discharge tubes, to prevent the gap t from being formed between the flange 7 of the cylindrical member 2 and one of the discharge electrodes 4, 8 requires enhancing the machining precision of both the cylindrical member 2 and the discharge electrodes 4, 8. This in turn results in a significant cost increase for manufacturing the discharge tube 1.